Fish oils are unique dietary fats which powerfully lower both fasting and postprandial triglyceride levels in humans. Although inhibition of hepatic triglyceride synthesis is widely believed to be the reason for the reduction in fasting triglycerides, the reason why omega3 fatty acids (FAs) lower chylomicron levels is virtually unknown. The purpose of this project is to study the mechanism, and in so doing to evaluate two new methods to explore chylomicron metabolism in humans. We will study both humans and rats to determine whether chronic omega3 FA supplementation enhances chylomicron clearance or slows fat absorption/chylomicron secretion. Healthy volunteers (n=24) will be given clinically-relevant doses (5 g/d) of fatty acid ethyl esters. They will take an olive oil placebo for 4 weeks and then be randomly and blindly assigned to take either safflower oil (omega6 PUFA control) or a fish oil concentrate (containing 64% EPA+DHA) for the next 4 weeks. During the placebo and the active treatment phases we will test the absorption hypothesis (by intestinal perfusion to measure fat absorption over a length small bowel, and stool collection after an oral fat load to look for fat malabsorption), and the clearance hypothesis (by injection of autologous, chemically-labeled chylomicrons to measure clearance kinetics, and measurement of endogenous lipoprotein and hepatic lipase activities). We will also determine the effect of omega3 FAs on fractional cholesterol absorption. Studies in rats will examine the same hypotheses, only more invasively. Fat absorption will be assessed by quantitative lymphatic drainage after a high fat meal and by accumulation of labeled chylomicrons in the plasma during lipolytic blockade using Triton WR1339+ethyl oleate. Homologous and heterologous chylomicrons will be injected into control and omega3 FA fed rats and their clearance rates determined. Chylomicrons will also be injected into functionally hepatectomized rats to assess how rapidly they are converted into remnants, and into isolated livers to determine their rate of hepatic remnant uptake. These studies should reveal the mechanism behind the remarkable postprandial hypochylomicronemia observed in humans taking omega3 FAs. They should also produce a new method for studying LpL activity in vivo in humans. Future studies will explore the molecular mechanisms responsible for the omega3 FA-induced reduction in chylomicron levels.